Category Archives: Spectroscopy

For three days in early October, the South Cloisters and Garden Room of UCL played host to a festival of astronomy – Your Universe. Organised by Francisco Diego and Farah Islam from the department of Physics and Astronomy, the event was divided into two school days and one Saturday for the general public. The events saw demonstrations and explanations of different aspects of astronomy delivered by researchers in the department as well as two lectures, one each on Friday and Saturday evening.
The quiet and darkness of the Garden Room saw three presentations. The first was a powerpoint presentation on exoplanets – planets orbiting stars other than our Sun – delivered by David Johnson. The second was the Magic Planet, a globe onto whose inner surface was projected the atmosphere or outer surface of the Sun, several planets and satellites in turn. Finally, a demonstration of spectroscopy and the fingerprints of colours emitted and absorbed by individual elements was given by Gleb with lamps and spectroscopes capable of splitting light into the rainbow of available colours.
In the South Cloisters, another five demonstrations lay in wait. Firstly Emily Hall expanded minds with a talk on cosmology, discussing Dark Matter, that mysterious thing that interacts only gravitationally with normal matter, and Dark Energy; the curious driving force behind the expansion of the universe. Next came a demonstration of robotic telescopes controlled over the internet from a NASA and Harvard maintained website. The third talk took in the life cycle of stars and an explanation of the HR diagram that astronomers use to categorise stars. The fourth demonstration was telescopes, including scopes either looking at the Sun in the light of hydrogen atoms or, during less clement weather, at postcards at the other end of the South Cloisters, and a display on the University of London Observatory, used by UCL students studying astronomy. Finally, a demonstration of the timescales involved in the creation of life and the universe rounded off the main set of events.
Outside of the main event, more sedate displays in the Octagon and North cloisters were within easy reach of guests. These included an orrery, showing the motions of planets and major satellites around the Sun and a book of satellite images open at plates showing the Earth and Moon as seen by Lunar Orbiter 5. In the North Cloisters, the entire length of the space had been taken up by Pete Grindrod’s display; a high resolution image, ten miles of the surface of Mars as seen by the HiRISE camera on the MRO satellite, presently in orbit of the Red Planet.
With eight groups of primary and eight secondary age pupils per day, we saw around five hundred school children pass through the displays during the first two days and more than another hundred members of the public on Saturday. They also enjoyed two lectures; Mikako Matsuura’s description of seeing star birth through the eyes of the Herschel infrared space telescope and Francisco Diego’s answer to the question of why have we not found evidence of aliens.

Gliese 581 is a name well known and repeated in exoplanetary circles. Lying 20.3 light years from us in the constellation of Libra, it is a red dwarf star now known to have at least six planets. Twice before, the star has been in the news with one of its planets declared close to the habitable zone – the slender band of orbits around the star where the stellar radiation is high enough to melt ice, but low enough not to boil water. Planet C was close, but on the hot side, too close to the star. Planet D was close, but on the cold side. Now a planet G has appeared and it is right in the middle of the habitable zone.

Appearing in this region around a star is not in any way proof that a planet can support life, it is merely a suggestion that it is likely to fulfill at least one of the prerequisites – the balance of radiation from its host star is sufficient to sustain liquid water. But further properties derived from observations of the star are quite positive. The planet is 3-4 times the mass of the Earth, suggesting a rocky planet with a defined surface, which at a similar density would put it at 1.2-1.4 Earth masses, giving a surface gravity not that dissimilar to our own. That would imply sufficient gravity to hold an atmosphere. But the similarities with the development of our own world do seem to end there.

The planet orbits its host star in 37 days. The low luminosity of Gliese 581 means that to be in the habitable zone in terms of radiation, the planet must crowd close to the star. This means the star has sufficient gravitational effect to tidally lock its satellite – meaning as with the Moon in orbit of the Earth, the same face of the planet always points toward the star. This would imply a searingly hot one face and freezing cold dark side, with winds racing from one side to the other. At the day night boundary, buffeted by these winds, more moderate climates would be seen at the different latitudes, where life could start, possibly evolving to take advantage of less temperate spaces on the planet after leaving the cradle. Red Dwarfs are long lived stars, so the time will also be there to do it.

But gravity and photons aren’t all a star can put out. Gliese 581 isn’t a flare star, that is, it isn’t known for sudden massive bursts of material from its surface, but it will still lose mass through slow stellar winds. It is believed that planets in close orbits to their host stars tend to lose their magnetic fields, or see them closely affiliated with that of their host stars. The protective magnetosphere, the magnetic sheath that protects us against the particle radiation that results from hot matter essentially expanding off the surface of our Sun, may not be replicated in this new place.

Whether or not there is life in this increasingly diverse place, the careful measurements carried out by the team, who measured the Doppler shifts of the light from the wobbling star to an amazing precision, do show that pulling out rocky planets in the habitable zones is within reach of modern technology. However, with the actual light from the planet lost in the glare of the star, it will take time and new technology to tease out the signal of life within that light, should it be there.

The paper announcing the discovery of the planet is here and one submitted at the same time detailing modelling of planet D’s atmosphere to determine whether or not its properties are sufficient to insulate the bitter cold and foster life is here.

Following Tom Stallard’s EPSC 2010 presentation on videos of the changes in Saturn’s aurora (including faint auroral signals) and linking that to events in the magnetosphere in general, there’s been a couple of news items in magazines. New Scientist managed to get the wrong end of the stick, believing the infrared aurora to be the new thing (it isn’t, just the video of it and the faintness of the signal) whereas Astronomy Now gave a more proper writeup, including a genuinely new thing from the same session on the detection of Saturn’s radio aurora. Radio emissions come from charged particles getting deflected by magnetic fields, which is what happens when auroral particles head to the auroral regions of a planet. Satellites have long been able to put themselves in the thin zone of emissions from Earth, Jovian observers have detected the Io current associated auroral radio signals, but Saturn has been more elusive to satellites in its region until now.

There’s a few probes out there, gathering data in the solar system, so starting from the inner planets, today’s news includes:

Venus Express has been watching a vortex playing in the atmosphere above the south pole of Venus. In 1979, the Pioneer Venus mission spotted a vortex above the north pole and on arrival in 2006, Venus Express found its southern twin. However, continuous recording of the phenomenon has shown that the double-eyed appearance of the vortex was simply a coincidence. Other vortices have since come and gone at the south, leaving the double eyed feature nowhere to be found. Full details are here.

Sticking with Venus Express, but delving lower into the atmosphere, lightning discharges on the second planet from the Sun have been confirmed as happening as frequently on Venus – one hundred times a day – as on Earth. The storms are strongest on the dayside, where the Sun provides the energy for cloud particles to collide and rub together, and also strongest towards the equator, for the same reason. The signals, previous detected by other probes using different instrumentation, were detected using Venus Express’s magnetometer and were apparent from the earliest times after insertion into orbit. Full details here.

Onto Earth now and a crater seen in satellite images bundled into the Google Earth software has been confirmed as being an impact feature. The feature was spotted in 2008 in images of the Egyptian desert and has been measured at 45 metres diameter and 16m deep. The crater was forged by the impact of a 1.3m meteorite weighing in at 10 tonnes (one tonne of which has now been collected up) sometime in the last several thousand years. The crater has evaded the geological processes that tend to erode such features and seems to have also escaped notice from human eyes in all that time. More on the discovery and confirmation of Kamil crater (including a google maps page showing the thing) can be seen here.

Three years of data from the SMART-1 mission to look at the Moon have been released by ESA. The three scientific instruments on board the probe were: the Advanced Moon micro-Imager Experiment (AMIE), which was a camera in visible and near infrared light, which watched the terrain changing as the shadows changed and so mapped the southern pole of the Moon to a resolution of 40m per pixel; the SMART-1 InfraRed Spectrometer (SIR), which watched the spectrum of the Moon in the 0.9-2.6 micrometer wavelength range, enabling mapping of pyroxene and olivine in solidified lunar magma exposed by asteroid impacts; the Demonstration of a Compact Imaging X-ray Spectrometer (D-CIXS), which mapped the Moon in the 0.5-10keV photon energy range, enabling x-ray reflection spectroscopy of some heavy elements. Fortunately for D-CIXS, a high energy solar flare provided additional x-ray flux enabling some of the elements that would normally be producing very dim signals to shine brightly enough to be confirmed. The data can be found here.

Further out and Mars Express has been used to examine the unusual behaviour of carbon dioxide ice in the Martian polar cap. Observations of the ice showed unusual behaviour as the cap receded in warmer times. The signal of the CO2 is seen to weaken and vanish as it sublimates from ice to gas, but then not long after, the signal suddenly returns before vanishing again. This fade in, it was hypothesised, could be due to a protective layer of dust or water ice protecting the underlying CO2. As there was no change in brightness, as there would be if white ice gave way to dark dust, researchers concluded water ice, invisible to the instruments they were using, must be the insulating layer. The Martian polar caps contain a mixture of water ice and CO2 ice. CO2 sublimates at a lower temperature, so what was happening was the exposed CO2 vanished, leaving a water ice shell (added to by condensing water ice from warmer, lower latitudes) and underlying CO2. Then there came the problem of why the water ice was suddenly stripped away revealing lots of CO2 to provide the second signal. Models of downward flowing winds created by the warming showed that these were capable of doing the stripping, lending the final piece of the theory. Full details here.

The Rosetta probe is set for a date with the Comet 67P/Churyumov-Gerasimenko in May 2014. Computer models of the three dimensional shape and motion of the comet have been used to assess what part of it will be least prone to outgassing as the block of rock and ice closes in on the Sun. The results suggest the southern hemisphere will be the best place for Philae, the lander delivered by the probe, to hook on and sample the comet material. Before the probe meets the comet, this hemisphere will receive the largest amount of sunshine, eroding the outer crust and exposing pristine material within. By the time the probe meets the comet, and after delivery of the lander, the north pole of the comet will be in the glare of the Sun, and so most prone to outbursts. The lander will use harpoons and jets to hook onto the comet during its studies. Full details here.

Cassini will be performing the first in a series of Titan flybys over the next eighteen months later today. The aim will be to supplement climate studies of the distant satellite of Saturn, more details here.

Cassini has also been taking a good look at the parent body in the infrared. Using the Visual and Infrared Mapping Spectrometer, Tom Stallard of Leicester (and formerly UCL) has been observing changes in the southern lights of Saturn to compare with other processes going on in Saturn’s magnetosphere, the aim being to connect the two. The aurora of Saturn are complex and involve both large scale motions of the magnetosphere – contractions and expansions caused by the uneven passage of the pulsed solar wind – and small scale structure such as disruption of particle and energy flows by the moons of Saturn inside the magnetosphere. More details of his infrared work are here and some images of ultraviolet auroral signals from Saturn and Jupiter by the Hubble Space Telescope are here.

Anti matter seems to be fairly well understood. We see it in cosmic ray collisions with particles in the upper atmosphere. We use it in PET scans in hospitals. We create it in the Large Hadron Collider to smash into ordinary matter and investigate the inner workings of both.

Antimatter is known to have the opposite electric charge and other properties to its ordinary matter equivalent. We know there’s more matter than antimatter, but don’t know why. But while we can investigate strong forces like electromagnetism, another mystery arises when considering a very weak force – gravity.

In most of the situations we see antimatter, it is zipping along at a great speed and its momentum is hardly affected by accelerations due to gravity. In most cases the deflection due to magnetic fields is too great for gravitational effects to be measured in the short life of the stuff. To put it bluntly, we’ve never really just let antimatter go and seen whether it goes up, down or stays put.

Virtual particles and antiparticles are constantly being created and destroyed in pairs. Hawking suggested that should this happen on the boundary of a black hole, the gravity should be sufficient to rip out one of the pair before it is attracted to its antiparticle and they annihilate. As such particle radiation would be detectable. Now other researchers have suggested that if antimatter does interact with mass differently to ordinary matter, the signature will be heavy in anti matter.

Their specific example was the creation and destruction of neutrinos, which are affected only by gravity. Close enough to a black hole and the matter one would be pulled in while the antimatter one, if the hypothesis is right, should be pushed out. The IceCube Neutrino observatory should be able to spot black holes shining bright in antineutrinos. However, there are other hypotheses that can produce such a signal, so even if the detectors did see such an event, they would need further work to determine if antimatter really does run away from mass.

The radio telescope, which has stations dotted around Europe, will combine signals in a process called interferometry to create a single large telescope operating in the 1-10m wavelength. This aperture synthesis will allow higher resolution images than an individual station would be able to achieve. This is necessary as resolution depends on wavelength and the diameter of a telescope. At metre wavelengths, the size of a telescope required is enormous. LoFAr’s arrangement is such that the combined signal will be equivalent to a telescope the size of the separation of the individual stations, while the sensitivity will be equivalent to the combined sensitivity of the observatories.

Aperture synthesis is routinely used in radio astronomy to counter the poor resolution of radio waves, but rarely on this scale. It has also been used in shorter wavelengths, but this is more difficult as the higher frequencies of light make adding the signals more complex. Telescopes like Keck and Subaru are designed to use large single observatories in pairs to achieve larger synthetic observatories in infrared and even optical light.

Each new wavelength regime observed opens a new chapter in astronomy. Higher energy events are seen at low wavelengths, with things as hot as stars and aurorae generally in ultraviolet and mostly optical regimes. Newly forming protostars, existing planets and asteroids are seen to glow brightest in the infrared and dust as well as the background hiss of the afterglow of the formation of the optically thin universe, the CMB, are seen in the microwave. Radio wavelengths most often show the emissions from charged particles deflected by magnetic and other fields. As such, the new telescope can be used to observe magnetic fields on the cosmic scale, the interaction of the solar plasma with objects in the solar system, cosmic rays hitting our atmosphere and processes involved in star formation and the growth of black holes, all of which involve or are revealed by the deflection of charged particles to a greater or lesser extent.